1. Field of the Invention
The present invention relates to a method of generating modulated optical signals, and to a source of modulated optical radiation, and in particular to the use of a semiconductor electroabsorption modulator in a radio over fibre communication system.
2. Related Art
Demand for broadband telecommunication services to the home and to small business is expected to increase dramatically over the next decade. Optical fibre and millimeter-wave radio are both individually capable of supporting the large bandwidth requirements associated with these services. Providing optical fibre cabling direct to the home or business is one way of providing high capacity, but for operational reasons this is not always an appropriate solution. Alternatively millimeter-wave radio systems, such as the RACE Mobile Broadband system or radio LANs, are flexible and offer the advantage of expedient provision.
In recent years a hybrid of these two technologies has been developed, and is termed radio over fibre. Radio over fibre systems utilise optical fibre transmission to deliver radio signals directly to a point of free space radiation, usually an antenna site. Dependent on the application of the radio over fibre system, the radio signals may be of VHF, UHF, microwave or millimeter frequency. In general, a radio over fibre communications system will comprise a first, or central site, where an optical signal having a radio frequency carrier is generated, a second, or remote site, linked to the first site by an optical fibre, the second site having a transmitting radio antenna, and a third site having a receiving radio antenna. Thus,, data encoded on the optical carrier at the first site is optically transmitted, via the optical fibre, to the remote site, transmitted as a free space radio signal from the remote site to the receiving radio antenna at the first site, and demodulated. By delivering the radio signal optically, via the optical fibre link, it is unnecessary to generate a high frequency radio carrier at the remote site, which is usually not easily accessible, and not in a benign environment. Optical fibre is an ideal transmission medium for this purpose due its low loss, high frequency, and wide band capability. The principle advantage of radio over fibre systems is their ability to concentrate most of the expensive, high frequency equipment at a centralised location, allowing the remaining equipment at the remote site to be simple, small size, lightweight, and low power consumption. This results in straight forward installation, low maintenance, and a range of simplified options for electrical power provision. The centralisation of high frequency equipment also increases operational flexibility and the potential for frequency reuse or sharing between a number of users. Furthermore, the radiation frequency can be controlled centrally, in an environment shielded from severe climatic variations, and therefore can be extremely stable.
A key issue for radio over fibre communications systems will be the efficient generation of radio frequency modulated optical signals, since in order to benefit from the principle advantage of radio over fibre systems, the radio frequency carrier, modulated onto the optical signal, must be suitable for direct retransmission by a transmitting radio antenna, without upconversion. While techniques and components exist for generating modulated optical signals at lower radio frequencies, e.g. 1 GHz to 15 or even 20 GHz, as the required radio frequency modulated optical signal increases in frequency it is increasingly difficult to generate these optical signals efficiently, and cost effectively. Higher frequency radio over fibre communication systems are particularly attractive for a number of applications such as Multipoint Video Distribution Services (MVDS), and Mobile Broadband Systems (MBS). Frequency bands in the millimeter-wave range have been allocated in Europe to both these applications at 40 to 42 GHz and 62 to 66 GHz respectively. The advantages for working in these higher frequency bands stem not only from the availability of these areas of the electromagnetic spectrum, but also from technical factors such as high antenna gain, small physical antenna size, and good frequency re-use resulting from high propagation losses beyond line of sight paths, and from atmospheric attenuation. If higher frequency radio over fibre communication Systems are to be commercially viable, then a low cost method of generating radio frequency modulated optical signals is essential.
There are number of known techniques for generating radio frequency, and in particular millimeter-wave, modulated optical signals. Although direct modulation of laser diodes is not feasible at millimeter wave frequencies, resonant enhancement of the laser frequency response, by use of an external cavity, can generate modulated optical signals at a frequency of 45 GHz see for example J B Georges, N H Kiang, K Heppell, M Sayed, and K Lau, "Optical Transmission of Narrowband Millimeter-wave Signals by Resonant modulation of Monolithic Semiconductor Lasers", IEEE Photonic Technology Letters, Vol 6, No. 4, pp 568-570, 1994. However resonant enhancement techniques fundamentally limit the bandwidth of any data signals to be carried by the communications system. Optical heterodyning is a technique that can give very efficient optical signal generation, but which requires complex stabilisation to achieve a beat signal with low phase noise, see for example D C Scott, D V Plant, and H R Fetterman "60 GHz Sources Using Optically Driven Heterojunction Bipolar Transistors", Applied Physics Letters, Vol 6, No. 1, pp 1-3, 1992. Harmonic generation techniques, in which harmonics of the electrical drive signal applied to an optoelectronic component are utilised, are attractive because they allow relatively low frequency, and thus low cost, optoelectronic components to be utilised, and because the purity of the optical signal is derived from the electrical drive signal. Both lasers and optical modulators have been used for harmonic generation of radio frequency modulated optical signals. An optical frequency modulated laser, in combination with a dispersive fibre was used by D Wake, I C Smith, N G Walker, I D Henning, and R D Carver, in "Video Transmission Over a 40 GHz Radio-fibre Link", Electronics Letters, Vol. 28, No. 21, pp 2024-2025, 1992. This technique suffers from the disadvantage that a controlled amount of dispersion must be present. Harmonic generation using Mach-Zehnder (HZ) modulators has also been demonstrated, for example by J J O'Reilly, P M Lane, R Heiderman and R Hofstetter, "Optical Generation of Very Narrow Linewidth Millimeter-wave Signals", Electronics Letters, Vol. 28, No. 25, pp 2309-2311, 1992.
In general when utilising harmonic generation, it is desirable to use the lowest possible harmonic, because higher order harmonics contain less power, and thus result in shorter maximum free space transmission distance. However, in order to utilise a lower order harmonic to generate a given free space transmission frequency, the harmonic generator must be designed to operate at a higher frequency. For semiconductor modulators this increases the complexity of the modulator design, and the cost of packaging the modulator. As the design frequency of a modulator is increased, small imperfections in the packaging have a greater effect on the performance of the modulator since their size approaches the wavelength of the electrical signals employed. In addition, any resonant modes of the package need to be carefully analysed and avoided if it is to be operated at higher frequencies. Furthermore, parasitics (such as bond wire inductance) and the loss of dielectric materials both affect the performance of the modulator at higher frequencies. Thus it is particularly important that an optical modulator employed for harmonic generation can efficiently generate harmonics, so that a low frequency design of modulator can nevertheless give sufficient power in a higher harmonic for free space transmission.